1. Technical Field
The present invention relates to photoelectronic sensors and, in particular, to a coaxial regressive reflection type photoelectronic sensor.
2. Related Art
A reflection type photoelectronic sensor that utilizes reflection of light is known as a photoelectronic sensor used to detect the presence of an object. The reflection type photoelectronic sensors include a so-called regressive reflection type photoelectronic sensor.
The regressive reflection type photoelectronic sensor generally includes a light projector/receiver incorporating a light emitting element and a light receiving element. When using such a sensor, a regressive reflection plate for reflecting the light emitted from the light projector/receiver and returning the light to the light projector/receiver is arranged facing the light projector/receiver. If an object does not exist on an optical path of the light emitted from the light emitting element, the light is reflected by the regressive reflection plate and entered to the light receiving element. If an object exists on the optical path, on the other hand, the light emitted from the light emitting element does not enter the light receiving element as the light is shielded by the object. In other words, an amount of received light at the light receiving element differs depending on whether or not the object exists on the optical path, and thus the regressive reflection type sensor detects the presence of the object based on the difference in the amount of received light.
The regressive reflection type photoelectronic sensor includes a twin-lens type and a coaxial type. In the case of the twin-lens type, the light projecting path and a light receiving path are physically separated. In the case of the coaxial type, the light projecting path and the light receiving path are almost coincided, and such optical paths are separated by an optical element (polarizing beam splitter, half mirror, etc.).
The twin-lens regressive reflection type photoelectronic sensor is disclosed, for example, in Japanese Unexamined Patent Publication No. 10-255611 (Patent Document 1). According to this document, the photoelectronic sensor includes a light projecting lens and a light receiving lens. The light projecting lens is arranged only in a region having a possibility the light ray transmitted through the light projecting lens enters the light receiving lens after being reflected by a reflector. The light receiving lens is arranged only in a region having a possibility the light ray exit from a light projecting portion enters the light receiving lens after being reflected by the reflector.
The coaxial regressive reflection type photoelectronic sensor is disclosed, for example, in Japanese Unexamined Patent Publication No. 10-255612 (Patent Document 2). According to Japanese Unexamined Patent Publication No. 10-255612, a slit for limiting the light projecting range is provided in a light projecting element to enhance the utilization efficiency of the light reflected from the regressive reflection plate and to prevent malfunction by refraction of a transparent object. The light projection beam is projected on the regressive reflection plate by a center portion of the lens. The proportion the light is not received at the light receiving element of the lights reflected by the regressive reflection plate is thereby reduced, and thus the utilization efficiency of the light is improved.